NTP 时间协议算法

      网络延时与时钟偏差的测量


Timestamp Name ID When Generated
Originate Timestamp T1 time request sent by client
Receive Timestamp T2 time request received at server
Transmit Timestamp T3 time reply sent by server
Destination Timestamp T4 time reply received at client

t 为服务器和客户端之间的时间偏差；d 为两者之间的往返时间
因为：T2=T1+t+d/2; T2-T1=t+d/2; T4=T3-t+d/2; T3-T4=t-d/2;
所以：d=(T4-T1)-(T3-T2); t=((T2-T1)+(T3-T4))/2